The present invention relates to a semiconductor device, and more particularly to a semiconductor device including an output transistor that performs switching of high current.
A power semiconductor device that performs switching of high current limits a current that flows through an output transistor when short-circuit or malfunction is caused in a load connected to an output terminal. The power semiconductor device prevents the destruction of the semiconductor device by limiting the current that flows through the output transistor. One example of a method of limiting a current that flows through an output transistor is disclosed in Japanese Unexamined Patent Application Publication No. 2005-260658.
FIG. 5 shows an application example that uses a semiconductor device disclosed in Japanese Unexamined Patent Application Publication No. 2005-260658 as a so-called high-side switch (configuration in which the semiconductor device is connected to a higher-potential side than a load). In a semiconductor device 200 shown in FIG. 5, a series circuit of a detection transistor 203 and a detection resistor R204 is connected in parallel to an output transistor 202 that supplies a load current to a load RL, and a detection current that is substantially proportional to the output current (load current) that flows through the output transistor 202 flows through the detection transistor 203 and the detection resistor R204. A protection transistor 204 is connected between the gate and the source of the output transistor 202, and the gate of the protection transistor 204 is connected to a node between the detection transistor 203 and the detection resistor R204.
A control signal 205 that turns on/off the output transistor 202 is input from an input terminal IN. In this application example, since a high-side switch is employed in which an N-channel transistor is used as the output transistor 202, the voltage Vin of the control signal 205 in ON state is boosted to be higher than the voltage of a power supply terminal VB by a charge pump or the like (not shown).
When the output current that flows through the output transistor 202 increases and the detection current that flows through the detection transistor 203 and the detection resistor R204 increases in accordance therewith, the voltage that is generated in both ends of the detection resistor R204 (gate-source voltage Vgs204 of the protection transistor 204) increases. When the Vgs204 exceeds the threshold voltage of the protection transistor 204, the protection transistor 204 turns ON and the gate-source voltage of the output transistor 202 is lowered, which decreases the output current. Since the detection current decreases in accordance therewith, negative feedback is applied to decrease the Vgs204, and the output current is limited when it becomes in an equilibrium state.
Accordingly, the semiconductor device 200 shown in FIG. 5 includes a current limiting function to limit the control voltage applied to the gate of the output transistor 202 (and the detection transistor 203) when overcurrent flows through the output transistor 202.
However, the semiconductor device 200 has a problem that, as will be described below, variations of the resistance value and the threshold voltage due to manufacturing variations of the detection resistor R204 and the protection transistor 204 cause variations of the current that should be limited (output current that flows through the output transistor 202).
First, description will be made of the current limiting operation of the semiconductor device 200 as shown in FIG. 5. When a conducting current of the output transistor 202 is denoted by I202, and a conducting current of the detection transistor 203 is denoted by I203, the relation shown in the expression (1) is substantially obtained, although some deviation is caused strictly due to a voltage drop in the detection resistor R204. In the expression (1), the area ratio of the output transistor 202 to the detection transistor 203 is set to 1000:1.Expression 1I202=1000·I203  (1)
The condition in which the current is limited is when the voltage decrease of the detection resistor R204 is equal to or more than the threshold value of the protection transistor 204. In summary, the relation between the threshold voltage Vt204 of the protection transistor 204 and the detection current I203 in which current limiting operation is started can be obtained by the expression (2). The symbol R204 in the expression (2) shows the resistance value of the detection resistor R204.Expression 2Vt204=I203·R204  (2)
From the above expressions (1) and (2), the current value I202 of the limited current of the output transistor 202 is determined by the expression (3).
                    Expression        ⁢                                  ⁢        3                                                                      I          ⁢                                          ⁢          202                =                  1000          ·                                    Vt              ⁢                                                          ⁢              204                                      R              ⁢                                                          ⁢              204                                                          (        3        )            
From the expression (3), it is understood that variations of the threshold voltage Vt204 of the protection transistor 204 and the resistance value of the detection resistor R204 increase variations of the current value of the output current I202 that is limited. For example, it is empirically observed that variations of Vt204 by ±30% and R204 by ±30% cause variations of the current value I202 by ±42%.
On the other hand, Japanese Unexamined Patent Application Publication No. 2001-168697 discloses a technique of suppressing variations of the output current that is limited by driving the gate of the output transistor by a constant current.
As shown in FIG. 6, a semiconductor device 100 is a so-called low-side switch (configuration in which the semiconductor device is connected to a lower-potential side than a load). In this semiconductor device 100, an output terminal OUT is connected to a power supply through a load RL, and a ground potential is supplied to a ground terminal GND. An output transistor 102 and a detection transistor 103 are connected in parallel. Further, an NMOS transistor 107 is connected in series with the detection transistor 103. An NMOS transistor 108 constitutes a current mirror circuit with the NMOS transistor 107. Further, an NMOS transistor 109 has a diode-connected configuration in which the gate and the drain are connected, and is connected to a node between gates of the detection transistor 103 and the output transistor 102. The NMOS transistor 109 that is diode-connected is connected to make the gate-source voltage of the detection transistor 103 equal to the gate-source voltage of the output transistor 102. In summary, if threshold voltages of the NMOS transistor 109 and the NMOS transistor 107 connected in series with the detection transistor 103 are equal to each other, the gate-source voltages of the output transistor 102 and the detection transistor 103 are equal to each other. Thus, the detection transistor 103 is able to accurately detect the output current that flows through the output transistor 102. Further, a constant current source 106 is connected to the gate of the detection transistor 103 and the gate of the output transistor 102 via the diode-connected NMOS transistor 109, and drives the detection transistor 103 and the output transistor 102 by a constant current.